Technical Field
The present invention relates to the field of solid oxide fuel cells, and more particularly to a medium and high-temperature (the operating temperature is 500° C. to 900° C.) carbon-air cell based on a solid oxide fuel cell and a CO2 separation membrane, which is an energy storage cell with carbon as fuel.
Related Art
Fuel cells, as an electrochemical energy conversion device, can continuously convert fuel into electricity, and have energy conversion efficiency greater than conventional thermal power generation and energy density higher than common energy storage cells, and thus attract broad attention of people. Among various fuel cells, solid oxide fuel cells (SOFCs), as a type of high-temperature fuel cells, have a prominent advantage of fuel diversity, and in addition to hydrogen, hydrocarbons, carbon hydroxide compounds, ammonia, CO, even solid carbon can be used as fuel of SOFCs. As a fuel of fuel cells, solid carbon has various advantages compared with other fuels: (1) having high energy density, heat released by combustion of carbon is 20.0 kWh L−1, much higher than that of many other widely used fuels such as hydrogen (2.4 kWh L−1), methane (4.0 kWh L−1), gasoline (9.0 kWh L−1) and diesel (9.8 kWh L−1); (2) having a very rich source, solid carbon can be obtained through pyrolysis of coal, petroleum coke, biomass or organic waste, especially coal is the most abundant fossil fuel reserved on the earth and accounts for nearly 60% of all fossil fuels; (3) being safe and nontoxic, the solid carbon has a very distinct advantage in storage, transportation and preparation compared with hydrogen; (4) compared with cells with hydrogen as a fuel, hydrogen embrittlement does not exist in cells with carbon as a fuel, so silver can be conveniently used as a sealant, and the cells are suitable for long-term operation.
For carbon-air cells constructed with carbon as a built-in fuel, the theoretical capacity may be up to 8935 mAh g−1, which is nearly 25 times of the theoretical capacity (365 mAh g−1) of current lithium cells with carbon as the pole, and is more than twice of the capacity of lithium cells with elemental silicon as the pole; moreover, the charging process (regeneration) of carbon-air cells can be easily and conveniently achieved by adding carbon fuel. Because a cathode of the cell is an air pole, and the capacity of the cell is almost not restricted by the cathode, the theoretical capacity of the cell can be 40% to 60% of carbon capacity, which is dozens of times of the capacity of current lithium-ion cells. Additionally, carbon is a very stable fuel, so that carbon-air cells are expected to make breakthroughs in achieving high-capacity and high-safety energy storage cells, but currently there are very few reports about carbon-air cells for energy storage, because electrochemical oxidation of carbon is very difficult and it is difficult to achieve electrochemical oxidation of carbon by a conventional low-temperature electrochemical process. Additionally, compared with a conventional fuel cell, an energy storage cell is a closed system, and the direct carbon fuel cell will generate CO2 in gas state, so that an additional CO2 separation system is required, thereby greatly increasing the complexity of the cell, and reducing the volumetric energy density and weight energy density of the cell.